Self-aligned near surface strap for high density trench drams

ABSTRACT

A method and structure for a dynamic random access memory device comprising a storage trench, a storage conductor within the storage trench, a lip strap connected to the storage conductor, and a control device electrically connected to the storage conductor through the lip strap. The trench contains a corner adjacent the control device and the lip strap and has a conductor surrounding the corner. The control device has a control device conductive region adjacent the trench and the lip strap and has a conductor extending along a side of the trench and along a portion of the control device conductive region. In addition, the device can have a collar insulator along a top portion of the trench, wherein the lip strap includes a conductor extending from a top of the collar to a top of the trench. The lip strap can also extend along a surface of the device adjacent the trench and perpendicular to the trench. A node dielectric, lining the trench where the lip strap surrounds an upper portion of the node dielectric, is adjacent the top portion of the trench and can have a trench top oxide where the lip strap extends into the trench top oxide and forms an inverted U-shaped structure. Further, the lip strap can include a conductor extending along two perpendicular portions of a top corner of the trench.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to conductive strapsutilized to connect storage devices to transistors and more particularlyto self-aligned near surface straps for high density trench storagedevices.

[0003] 2. Description of the Related Art

[0004] Conventional systems for processing trench dynamic random accessmemory (DRAM) structures form a buried strap by performing two recesssteps and etching the collar after the first of these recesses. Thesecond recess provides margin for a dielectric on top of the trench toisolate it from the passing wordline. In this scheme, the buried strapdepth (determined by the sum of the first recess and collar wet etch)cannot be made very shallow and is thus not scalable from standarddevice scaling rules, which as is well known in the art, require shallowjunctions. Secondly, the difference in two recess depths combined withthe required presence of a thin interface (to prevent dislocations)places severe constraints on the tolerance of the resistance contributedby the buried strap.

[0005] Several suggestions have been made to ameliorate some of theproblems mentioned above including methods to form the buried strapafter active area (AA) definition (which adds considerable cost), adivot strap (which can cause a large pad oxide undercut), and a surfacestrap (which places considerable pressure on isolation planarization).Therefore, there is a need for a strap that allows better control of theleakage of current from the capacitor and is thus much more scalable.

SUMMARY OF THE INVENTION

[0006] It is, therefore, an object of the present invention to provide astructure and method for a dynamic random access memory devicecomprising a storage trench, a storage conductor within the storagetrench, a lip strap connected to the storage conductor, and a controldevice electrically connected to the storage conductor through the lipstrap. The trench contains a corner adjacent the control device and thelip strap and has a conductor surrounding the corner. The control devicehas a control device conductive region adjacent the trench and the lipstrap and has a conductor extending along a side of the trench and alonga portion of the control device conductive region. In addition, thedevice can have a collar insulator along a top portion of the trench,wherein the lip strap includes a conductor extending from a top of thecollar to a top of the trench. The lip strap can also extend along asurface of the device adjacent the trench and perpendicular to thetrench. A node dielectric, lining the trench where the lip strapsurrounds an upper portion of the node dielectric, is adjacent the topportion of the trench and can have a trench top oxide where the lipstrap extends into the trench top oxide and forms an inverted U-shapedstructure. Further, the lip strap can include a conductor extendingalong two perpendicular portions of a top corner of the trench.

[0007] The invention further includes a method of forming a dynamicrandom access memory structure for forming a trench within a substrate,filling the trench with a trench conductor, forming a pad oxide along asurface of the substrate adjacent the trench, forming a collar along anupper portion of the trench such that the collar insulates the substratefrom the trench conductor, recessing the collar and the pad oxide,depositing a lip strap over the trench conductor and in recessesproduced by the recessing, and forming an isolation region adjacent thelip strap.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The foregoing and other objects, aspects and advantages will bebetter understood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

[0009]FIG. 1 is a schematic diagram depicting a DRAM structure in theformation of the invention;

[0010]FIG. 2 is a schematic diagram depicting a DRAM structure in theformation of the invention;.

[0011]FIG. 3 is a schematic diagram depicting a section of the collarremoved;

[0012]FIG. 4 is a schematic diagram depicting a near surface strap;

[0013]FIG. 5 is a schematic diagram depicting a layer of trench topoxide and pad pull back;

[0014]FIG. 6 is a schematic diagram depicting a completed structureaccording to the invention;

[0015]FIG. 7 is a schematic diagram depicting another lip strapaccording to the invention;

[0016]FIG. 8 is a schematic diagram depicting a different aspect of theinvention; and

[0017]FIG. 9 is a schematic diagram depicting yet another aspect of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0018] This invention comprises a method and structure relating toconductive straps utilized to connect storage devices to transistors andmore particularly to self-aligned near surface straps for high densitytrench storage devices, the structure is referred to herein as a nearsurface or ‘lip’ strap. The near surface strap is used in the DRAMstructures of semiconductor chips or devices and replaces theconventional buried strap that is typically used with DRAM structures.

[0019] The inventive near surface strap provides an electricalconnection between a conductor or capacitor in the trench of a DRAMstructure and the active devices on the semiconductor chip. While thisdisclosure will make obvious to those skilled in art other advantages,one benefit of the invention is the formation of a connection betweenthe strap and the conductor that is at the surface of the semiconductordevice. This surface connection allows better control of leakage currentand, as a result, is much more scalable.

[0020] Several embodiments of the invention are discussed below. A firstembodiment is shown in FIGS. 1-6. Conventional DRAM processing wellknown to those ordinarily skilled in the art is performed until thestructure depicted in FIG. 1 is formed. The structure includes asubstrate 10 (e.g., silicon) having a trench 11 therein. The upperportion of the trench 11 includes an insulating collar 12 (e.g., such asan oxide). A pad oxide 14 extends horizontally from the collar 12 and ispositioned between the substrate 10 and a pad 16 which is formed from,for example, silicon nitride. Lining the trench is a node dielectric 18(e.g., silicon nitride). A conductor 20 (e.g., tungsten, polysilicon,)fills the trench 11. A shallow recess 22 is formed in the conductor 20down to approximately 20 nm below the upper level 28 of the substrate10. The recess 22 prevents the conductor 20 from extending above thesubstrate 10 surface.

[0021] Initially, the invention pulls back (e.g., etches) the nodedielectric 18 and the pad 16 so that the collar 12 is exposed, as shownin FIG. 2. While the figures illustrate a LOCOS collar, as would beknown by one ordinarily skilled in the art, the invention may be easilyextended to a conventional TEOS deposited collar. Then, as shown in FIG.3, a strap recess 24 is formed using a wet etch. The oxide may also beetched with anisotropic Reactive Ion Etch (RIE) selective to the padnitride (16) and silicon (20 and 10). In a preferred embodiment, thestrap recess 24 encompasses approximately 20 nm in width and extendsfrom the pad oxide 14 to below the top portion of the node dielectric18. If the pad oxide 14 has sufficient thickness, the wet etch maycreate an undercut in the pad oxide 14. In any case, the subsequent fillprocess discussed below will completely fill any undercut.

[0022] As shown in FIG. 4, the lip strap 26, which can be formed of anysuitable conductor, such as polysilicon, tungsten, etc., is depositedand recessed below the pad 16 surface, and above the substrate surface28. In a preferred embodiment, the “lip strap” 26 extends approximately20 nm above the upper surface 28 of the substrate 10 as shown by item 27in FIG. 4.

[0023] The shape/size of the lip strap 26 and its height 27 above thesubstrate surface 28 is governed by the recessing of the lip strap 26.In a preferred embodiment the height 27 of the strap recess iscontrolled to guarantee that the strap 26 is above the substrate surface28. When determining the strap recess height 27, the pad 16 thicknessvariation is important and is considered by the invention. Theappropriate strap recess height 27 may also be controlled using alayered deep trench hardmask process that preserves some oxide (e.g., ina LOCOS process), as would be known by one ordinarily skilled in the artgiven this disclosure.

[0024] The pad is pulled back an additional amount as shown in FIG. 5,using similar processes as discussed above. In one embodiment, anoptional thin trench top (TT) layer 30 (e.g., silicon nitride, etc.) maybe deposited on the lip strap 26 to provide a margin for insulation froma passing gate (not shown) on the surface of the pads 16.

[0025] As shown in FIG. 6, a trench top oxide 32 such astetraethylorthosilicate (TEOS), high density plasma (HDP) oxide, orother suitable insulator, is formed. The trench top oxide 32 is thenpolished using chemical mechanical polishing (CMP) or other suitableprocess to planarize the upper surface of the structure.

[0026] As would be apparent to one ordinarily skilled in the art giventhis disclosure, the structure can be completed and the active area canbe defined using conventional shallow trench isolation (STI), raisedshallow trench isolation (RSTI), local the oxidation of silicon (LOCOS),or other similar process.

[0027]FIG. 7 illustrates one embodiment of the inventive structure afterthe formation of the isolation region and the gate conductor stacks.More specifically, FIG. 7 illustrates the lip strap 26, which ismanufactured as discussed above. The resulting RSTI region 70 definesthe active area and joins the TTO 32. The pad oxide 14 is replaced witha gate oxide 78, using processes well known to those ordinarily skilledin the art. The doped source/drain region is illustrated as item 77.Further, the gate conductor stacks include the gate conductor 74, theword line 75 and the nitride caps 76. An insulator 71 such as boronphosphorous silicate glass (BPSG) provides insulation and support. Thestructure is topped by another insulator, such as TEOS 72.

[0028]FIG. 7 also illustrates insulating spacers 73, 79. An importantfeature of the invention is that the insulating spacer 79 is formedadjacent to the TTO region 32. The spacer formation technology iswell-known to those ordinarily skilled in the art and will not bediscussed in detail here. However, the dielectric spacer 79 can beutilized in a number of different ways to produce different benefits forthe invention. More specifically, in one embodiment, the spacers 79 canbe formed after the gate sacrificial oxide is stripped and before thegate oxide 78 is grown.

[0029] Alternatively, the dielectric spacer 79 can be formed before thepad oxide 14 is stripped. This alternative process would allow thespacer 79 to be densified by the subsequent gate sacrificial oxidegrowth. This is especially true if the insulating spacer 79 is TEOSbased. Further, in an additional embodiment, the spacer 79 could be madeof phosphoro silicate glass (PSG) or doped glass. Therefore, in such asituation, the spacer 79 could provide additional dopants to the nodejunction if needed. Additionally, it is possible to form the spacer 79as a doped glass spacer after the pad oxide 14 is removed. Then, thedopant could be driven into the node junction when the gate sacrificialoxide is grown. At this point, the doped insulator 79 could be replacedwith a pure insulator, if desired.

[0030] It is to be noted that the structure in FIG. 7 may be achievedusing several methods. For example, after the top of the trench isfilled with TTO, the shallow trench isolation (STI) and fill processesare performed. The top of the trench may be filled concurrently as theSTI. The pad nitride is then shipped, spacers 79 are formed, the gatesacrificial oxide is grown, implants performed, the sacrificial oxide isshipped, the gate oxide is grown, and the gate polysilicon is depositedand polished to the TTO (32). The rest of the gate stack is depositedand the rest of the processing is conventional.

[0031] In another example, the structure in FIG. 7 can be formed usingthe following process. After the top of the trench is filled with TTO asin FIG. 6, the pad nitride is shipped, spacers 79 are formed, thesacrificial oxide gate is grown, implants performed, the sacrificaloxide is stripped, the gate oxide is grown, and the gate polysilicon isdeposited and polished to the TTO (32). A fresh pad nitride is thendeposited and STI is formed and filled. The pad nitride is stripped, therest of the gate stack is deposited and the conventional processing isthen performed to complete the structure.

[0032] Another embodiment of the invention is illustrated in FIGS. 8 and9. FIG. 8 illustrates a structure which is similar to that shown in FIG.1; however, the conductor 20 is recessed slightly below the uppersurface of the nitride pad 16, as shown by item 80. This is done toretain the conductor during the formation of the STI region 93. Then, asshown in FIG. 9, the shallow trench isolation region 93 (and optionalliner 92) are formed using conventional processes and the processesdiscussed above. After the STI region 93 is formed, the conductor isrecessed to level 90 and the lip strap 91 is formed, as discuss above.

[0033] Therefore, this embodiment of the invention is substantiallysimilar to the previous embodiments. However, in this embodiment, theSTI region is formed before the lip strap 91. By forming the lip strap91 after the STI region 93, the number of thermal cycles which the lipstrap 91 will receive is reduced. Each thermal cycle releases dopantsfrom the conductive lip strap 91. By reducing the number of thermalcycles to which in the lip strap 91 is subjected, more dopants areretained within the lip strap 91. This makes the lip strap electricallyshallower and increases the device performance accordingly.

[0034] Thus, as most clearly shown in FIG. 7, the invention is a dynamicrandom access memory device that includes a storage trench, a storageconductor 20 within the storage trench, a lip strap 26 connected to thestorage conductor 20, and a control device 77 (e.g., transistor)electrically connected to the storage conductor 20 through the lip strap26. The lip strap 26 is a conductor that extends along a side of thetrench and along a portion of the transistor 77 conductive region (e.g.,source/drain) and, therefore, surrounds a top corner of the trench. Thelip strap 26 extends from a top of the collar 12 to a top of the trenchand along a surface of the device adjacent the trench and perpendicularto the trench. In other words, the lip strap 26 extends along twoperpendicular portions of the top corner of the trench.

[0035] As shown in FIG. 6, the lip strap 26 surrounds an upper portionof the node dielectric 18 adjacent the top portion of the trench 11.Also, the lip strap 26 extends into the trench top oxide 32 and forms aninverted U-shaped structure within the trench top oxide 32.

[0036] A major advantage of the last embodiment is that the buried strap26 is formed after the AA oxidation and thus the invention has a reducedstrap out diffusion. All of the embodiments of this invention protectthe active silicon in the trench top region from the HDP oxide STI fill.Also, there is no risk of autodoping from the ‘lip’ to the neighboringactive area with the invention. Further, with the invention, the lipstrap 26 connection allows better control of leakage current and, as aresult, is much more scalable. Further, the processing discussed abovethe allows the lip strap 26 to be self-aligned, thereby avoiding theinaccuracies and problems associated with lithographic techniques.

[0037] While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. A dynamic random access memory device comprising:a storage trench; a storage conductor within said storage trench; a lipstrap connected to said storage conductor; and a control deviceelectrically connected to said storage conductor through said lip strap.2. The device in claim 1, wherein said trench has a corner adjacent saidcontrol device and said lip strap comprises a conductor surrounding saidcorner.
 3. The device in claim 1, wherein said control device includes acontrol device conductive region adjacent said trench and said lip strapcomprises a conductor extending along a side of said trench and along aportion of said control device conductive region.
 4. The device in claim1, further comprising a collar insulator along a top portion of saidtrench, wherein said lip strap comprises a conductor extending from atop of said collar to a top of said trench, said lip strap furtherextending along a surface of said device adjacent said trench andperpendicular to said trench.
 5. The device in claim 4, furthercomprising a node dielectric lining said trench, wherein said lip strapsurrounds an upper portion of said node dielectric adjacent said topportion of said trench.
 6. The device in claim 1, further comprising atrench top oxide, wherein said lip strap extends into said trench topoxide and forms an inverted U-shaped structure.
 7. The device in claim1, wherein said lip strap comprises a conductor extending along twoperpendicular portions of a top corner of said trench.
 8. A method offorming a dynamic random access memory structure, said methodcomprising: forming a trench within a substrate; filling said trenchwith a trench conductor; forming a pad oxide along a surface of saidsubstrate adjacent said trench; forming a collar along an upper portionof said trench such that said collar insulates said substrate from saidtrench conductor; recessing said collar and said pad oxide; depositing alip strap over said trench conductor and in recesses produced by saidrecessing; and forming an isolation region adjacent said lip strap. 9.The method in claim 8, further comprising forming a control deviceadjacent said trench, wherein said trench has a corner adjacent saidcontrol device and said lip strap comprises a conductor surrounding saidcorner.
 10. The method in claim 8, wherein said forming of said controldevice includes forming a control device conductive region adjacent saidtrench and said lip strap comprises a conductor formed along a side ofsaid trench and along a portion of said control device conductiveregion.
 11. The method in claim 8, further comprising forming a collarinsulator along a top portion of said trench, wherein said lip strapcomprises a conductor formed to extend from a top of said collar to atop of said trench, said lip strap further extending along a surface ofsaid device adjacent said trench and perpendicular to said trench. 12.The method in claim 11, further comprising lining said trench with anode dielectric, wherein said lip strap surrounds an upper portion ofsaid node dielectric adjacent said top portion of said trench.
 13. Themethod in claim 8, further comprising forming a trench top oxide, suchthat said lip strap extends into said trench top oxide and forms aninverted U-shaped structure.
 14. The method in claim 8, wherein said lipstrap comprises a conductor formed along two perpendicular portions of atop corner of said trench.
 15. A method of forming a dynamic randomaccess memory structure, said method comprising: forming a trench withina substrate; filling said trench with a trench conductor; forming a padoxide along a surface of said substrate adjacent said trench; forming acollar along and upper portion of said trench such that said collarinsulates said substrate from said trench conductor; forming anisolation region adjacent said trench conductor; recessing said collarand said pad oxide; and depositing a lip strap over said trenchconductor and in recesses produced by said recessing.
 16. The method inclaim 15, further comprising forming a control device adjacent saidtrench, wherein said trench has a corner adjacent said control deviceand said lip strap comprises a conductor surrounding said corner. 17.The method in claim 15, wherein said forming of said control deviceincludes forming a control device conductive region adjacent said trenchand said lip strap comprises a conductor formed along a side of saidtrench and along a portion of said control device conductive region. 18.The method in claim 15, further comprising forming a collar insulatoralong a top portion of said trench, wherein said lip strap comprises aconductor formed to extend from a top of said collar to a top of saidtrench, said lip strap further extending along a surface of said deviceadjacent said trench and perpendicular to said trench.
 19. The method inclaim 11, further comprising lining said trench with a node dielectric,wherein said lip strap surrounds an upper portion of said nodedielectric adjacent said top portion of said trench.
 20. The method inclaim 15, further comprising forming a trench top oxide, such that saidlip strap extends into said trench top oxide and forms an invertedU-shaped structure.